Odor control in wastewater treatment processes is typically focused on the prevention and elimination of hydrogen sulfide. Hydrogen sulfide is a colorless and odorous gas created in wastewater collection systems by the biochemical reduction of sulfate. Sulfate is a naturally occurring substance present in most well water supplies, dissolved naturally therein through geologic contact. Sulfate reducing bacteria are commonly present in wastewater collection and treatment systems. They convert sulfate to sulfide ions in wastewater systems deficient of dissolved oxygen, a condition typically occurring when wastewater is pumped long distances. Sulfide ions in turn react with excess hydrogen ions to form hydrogen sulfide gas which has an offensive odor. Humans can sense hydrogen sulfide at very low levels in air. Hydrogen sulfide is particularly offensive in concentrations near about 10 ppm and can cause respiratory paralysis if in excess of about 1000 ppm.
Nitrate salts, such as those of sodium and calcium, are commonly used for odor control in wastewater collection and treatment systems. Nitrate salts may generally offer an alternative source of oxygen to bacteria for biochemical metabolism, such that sulfates will not be reduced, and may also be effective in removing hydrogen sulfide. Liquid solutions of calcium nitrate and/or sodium nitrate may be injected into a wastewater collection system to remove and control hydrogen sulfide downstream. Nitrate salts can also be used to remove existing sulfide present at the point of addition of nitrate material. The nitrate is generally reduced by a denitrification process to odorless nitrogen gas, which may be safely emitted from the wastewater treatment system.